Vehicles are commonly equipped with safety restraint systems, such as air bag systems, to protect passengers, including a driver and a front seat passenger, for example, in the case of an accident. Since it can be assumed that the driver's seat of a vehicle is occupied when the vehicle is in motion, it is generally desirable that an airbag on the driver's side be deployed if the vehicle is subjected to a deceleration above a certain threshold. However, the driver is often alone in the vehicle so that deployment of the passenger side air bag is unnecessary. In other cases, it is desirable to deactivate the passenger side air bag when the passenger seat is occupied by a child or rear-facing child safety seat, or when passengers have leaned forward prior to the moment of activation.
Accordingly, several techniques have been developed for determining an occupancy status of a vehicle seat for purposes of deactivating air bag systems, such as those situations described above. One such technique involves placing electrodes in the vehicle seat to sense the capacitance of an object positioned on the seat between the two electrodes, with a low capacitance indicating an object having a low dielectric constant, such as a passenger. However, conventional capacitive sensing systems ignore a direct coupling capacitance between the electrodes which may lead to inaccurate measurement data and undesirable ground dependent results, such as when an occupant bends forward on a seat, for example.